As the demand for oil and gas resources becomes higher increasingly in China, more and more deep-wells, ultra-deep wells, and shale gas wells are drilled. Thus, the performance of drilling fluids must meet more demanding requirements. Wherein, the problem of well wall stability in the well drilling process has always been a challenge not solved completely yet in the world, and downhole complexities and accidents such as well caving, shrinkage, wedging of drilling tools, and bit balling, etc. resulted from well wall instability cause severe economic loss every year in the petroleum industry.
To solve the problem of well wall stability, at present, oil-based drilling fluid and high-performance water-based drilling fluid systems are mainly used in China and foreign countries. Oil-based drilling fluids have drawbacks including high cost, severe environmental pollution, and poor well-cementing effect, etc., though they can effectively stabilize well walls. As for high-performance water-based drilling fluids, the present design concept in the art is to select or design drilling fluid systems that can maintain the effective stress in mud shale as far as possible, merely in consideration of the aspect of “preventing the drilling fluid from causing hydrated swelling of the clay in the mud shale”. However, owing to the fact that there is no water-based drilling fluid system that can completely inhibit the hydration of clay, the well wall instability problem can't be solved. Hence, water-based drilling fluids can't replace oil-based drilling fluids to meet the requirement of well drilling under complicated geological conditions in unconventional oil and gas reservoirs. Since mud shale well wall instability is a result of mechanical-chemical coupled action, the development must starts from the aspect of enhancing the cohesive force in the mud shale of well wall (i.e., well wall strengthening), with consideration of chemical clay inhibition and micro-pore plugging in the mud shale, in order to effectively solve the problem of well wall instability. In view that the conventional drilling fluid systems can't attain the purpose of strengthening the well wall in the drilling process, it is urgent task to seek for a drilling fluid system that can effectively stabilize the well wall with a synergistic effect of strengthening, plugging, and inhibition.